MANUFACTURE OF BASIC METALS # 19

A metal finishing facility in Tunisia was subjected to pollution prevention diagnostic audit under EP3 program. This facility performs zinc, nickel, brass and chrome plating. The operations in the facility can be divided into five main steps:

1. Polishing: Manual polishing takes place in a separated room. The electric polishing machines is operated by two to three workers. It polishes bars and other large pieces before they are plated. The paste comes in brick form and consists of aluminum, silica and fatty acids that act as binders. The fatty acids that must be removed in the cleaning process. Dust generated by the polishing process is collected using vacuum cleaners connected to each machine.
2. Cleaning: Prior to metal finishing many parts are cleaned in a vapor degreaser that uses trichloroethylene (TCE) to remove grease and other impurities. Parts are removed from the degreaser, dried with paper towels and tied to racks before being placed in a queue for plating. The degreaser is heated so that TCE vapours rather than the liquid remove the grease. However, the degreaser does not currently function in this manner, and parts are placed in baskets, dipped in the liquid and removed. There is considerable drag-out from this operation, and worker exposure to TCE is quite high.
3. Racking: The facility electroplates various kinds of parts. Small pieces are plated in baskets that are placed directly in the solutions. Other parts are hung on special racks that are specifically constructed. After plating, the racks are dismantled, and the copper wire and excess plating solution that forms on the metal rack and wire are removed and collected for resale.
4. Metal finishing: The facility electroplates three different metals. Approximately 60 percent is brass, 30 percent is steel, and 10 percent is zamak. The metal finishing line consists of washing tanks, rinsing tanks, and nickel and chrome plating baths, and recuperation baths. A copper cyanide bath is located across from the line and is used to plate the zamak before it is plated with nickel and chrome. All the plating operations are manual. There is considerable variation in time taken in soaking operations. Workers generally place the parts first in a degreasing bath that contains caustic and cyanide in a heated solution. Parts are then rinsed in a continuous rinsing tank and placed in a stripping/pickling bath. Workers quickly rinse the part in a static rinse bath, place the part in an acid (H₂SO₄) bath, dip it in a continuous rinse tank, and then place it in an electrolytic washing tank that contains caustic and cyanide. They dip the part in a second continuous rinse tank, a second acid bath, and a final rinsing bath before plating. For plating, the parts are placed first in the nickel bath and then rinsed in a recuperative bath, dipped in a continuous rinse tank, and placed in a chrome bath. Following chrome plating, parts are dipped in a recuperative bath and then another continuous rinse bath. Sometimes, depending on how the piece looks, workers dip the part in a second rinsing bath before placing the part on a rack for drying.
5. Gilding: Gilding takes place in a separate room. Parts are rinsed in special rinse baths before gilding. Parts are plated with nickel before gilding

The facility faced various pollution problems including:

Cleaner Production Principle

Housekeeping; Process modification

Cleaner production recommendations were identified for six unit operations:

1. Polishing: Reduce time between buffing and cleaning; replace polishing compound with a compatible set of aqueous alkaline cleaners; improve operator practice by purchasing fixtures and jigs; and reduce compound and wheel use through proper operator practice and training.
2. Solvent degreaser: Replace this process step with alkaline cleaner.
3. Alkaline cleaner: Eliminate cyanide use and improve process solution monitoring.
4. Acid dip: Isolate acids for steel and brass, eliminate the process of depassivation of nickel, replace the process of mixed acid stripper with solutions in small tanks and practice segregation and recovery as well as improve process solution monitoring and control.
5. Plating: Capture and return 100% of drag-out to process solutions, clean baths less frequently and improve process solution monitoring and control.
6. Rinsing: Add agitation and sprays, control water use by turning off water while not used.

The cleaner production applications would result in,

Cost details are presented in table below:

None reported.

Contacts

Environmental Pollution Prevention Program

EP3 Tunisia

75 Avenue Mohamed V

1002 Tunis - Belvedere

Tunisia

Tel : + 216 (1) 788244 or 786680; Fax : + 216 (1) 787 245

Email: Balih@EP3.TN

 

ECOMED,

Agency for Sustainable Development of the Mediterranean,

Via di Porta Lavernale, 26, 00153 Roma,

Italy,

Tel : +39 6 5783564; Fax : +39 6 5781448

This case study was submitted to UNEP IE by: ECOMED, Agency for Sustainable Development of the Mediterranean, Via di Porta Lavernale, 26, 00153 Roma, Italy. It was reviewed by UNEP IE November, 1995.

Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.

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